Contents
Introduction
Advanced features
Background
Gasifier, prime mover and liquefier performance
carbon capture gasifier w/liquefier and electric generation
carbon capture gasifier with thermo-chem H2
stationary prime movers w/cryo-compression
stationary air liquefier
motor vehicle prime movers w/cryo-compression
motor vehicle air liquefier
Nomenclature
References
A viable carbon neutral energy storage concept, utilizing heat source and heat sink (cryo) fluids for transport and electric generation, is described. Near zero carbon release is due to pre or post combustion carbon capture during fuel gasification, and to high (n ~80%) thermal efficiency of the cryo-compression engine with subsequent lower demand for synthesis of fuel. Reduction of heat sink temperature to below ambient increases thermal efficiency by more than an equal increase in working fluid temperature of a prime mover, in conjunction with use of recovered energy to produce and deliver cryogenic heat sink coolant for heat rejection. Four prime mover cycles will illustrate this; cryo-compression piston expander (CCPE), cryo-compression internal combustion engine (CCIC), cryo-compression gas turbine (CCGT), and cryo-compression fuel cell (CCFC), in which estimated thermal efficiency increases from 25 to 70%, from 15 to 50%, from 25 to 70%, and from 50 to 80%, respectively. Hydrogen synthesis will ultimately be from thermo-chemical reaction with a "third way" heat source; recovered heat of gasification, instead of problematic solar or nuclear.
The term "cryo-sink/heat source storage" better describes "liquid air energy storage (LAES)", because a heat source (fuel or recovered) is also part of storage. Application of stored energy to the working fluid of a prime mover then involves maintaining temperature difference between heat source and cryo-sink temperatures. In stationary application off-peak grid, wind, or solar energy drives air liquefaction and may be supplemented by recovered energy of the system, as available. In motor vehicle application recovered energy of engine exhaust gas and regenerative braking drives air liquefaction during highway driving and urban driving, respectively, and may be supplemented by optimum efficiency load shifting between engine and liquefier, or by cryogen stored on the vehicle. Injection of liquid air (lqa) into the cryo-compressor of a reciprocating expansion engine, gas turbine or fuel cell delivers quasi-isothermal compression with least consumption of fuel and of liquid air. Highest driving performance of a compact car is provided by a cryo-compression fuel cell with exhaust gas recovery; estimated gasoline equivalent mileage exceeding 64 km/l (150 mpg) at 80 km/h (50 mph), while thermal efficiency of ~80% reduces required hydrogen storage to ~25% as with a conventional compact car. Highest electrical performance for a single family house is provided by a cryo-compression fuel cell with exhaust gas recovery, which consumes ~40% of fuel as with grid delivered power.
Hydrogen from wood gasification is the preferred heat source, in which a pre or post combustion process enables simultaneous CO2 capture. A gasifier can be universally deployed, recovering waste heat to drive synthesis of thermo-chemical hydrogen, while pressure let-down supplements air liquefaction. The concept takes advantage of the availability of commercial gasifiers and of those retired due to conversion of integrated gasification combined cycle (IGCC) plants to natural gas.
Advanced features
Innovations of cryo-sink/heat source storage include:
1. Fuel gasifier with heat recovery for electric generation, to raise steam for gasification, or to a thermo-chemical hydrogen reactor, and with synagas/nitrogen pressure recovery to drive air liquefier.
2. High efficiency H2 fired cryo-compression prime movers with exhaust gas energy recovery including internal combustion engine, fuel cell and gas turbine.
3. Stationary generator with cryo-compression engine featuring air liquefier powered by energy recovery of engine exhaust gas supplemented by off-peak grid, wind, solar, etc.
4. Motor vehicle with cryo-compression engine featuring air liquefier powered by energy recovery of engine exhaust gas supplemented by regenerative braking and load shifting to liquefier.
Energy storage is dominated by electro-chemical batteries, which have limited charge capacity especially below freezing, require substantial charge time, have environmental issues with disposal and mining component minerals, are hazardous and are dead-weight. Advantages of cryo-sink/fuel source storage over battery storage include:
* four times specific storage capacity,
* long service life with no disposal issue,
* consistent efficient performance,
* less hazardous in terms of toxicity and fire safety,
* global availability of air,
* long service life with no disposal issue,
* consistent efficient performance,
* less hazardous in terms of toxicity and fire safety,
* global availability of air,
* no special material requirements, and
* low capital cost.
References
Background
A liquid air car was first proposed in 1899 and 75 years later, with the 1970's oil crisis, a "liquid nitrogen economy" was proposed [1]. Some engines with liquid air working fluid were built and tested in the 1990's, including a fired gas turbine [2], and two air turbines [3, 4]. Subsequently, liquid air and nitrogen storage began gaining acceptance as indicated by a reciprocating liquid air engine, operating pilot plants of up to 5 MWe, development of a peaking turbine and of a 50 MWe commercial plant in the UK [5]. LAES plants will benefit from heat recovery from advanced fuel gasifiers [6] to liquefy air, in addition to solar and wind etc. Integrated gasification combined cycle [7] plants with fuel gasifiers have experienced a period of strong growth [8], which was subsequently interrupted by decommissioning and change-over to natural gas due to high carbon release.
A liquid air car was first proposed in 1899 and 75 years later, with the 1970's oil crisis, a "liquid nitrogen economy" was proposed [1]. Some engines with liquid air working fluid were built and tested in the 1990's, including a fired gas turbine [2], and two air turbines [3, 4]. Subsequently, liquid air and nitrogen storage began gaining acceptance as indicated by a reciprocating liquid air engine, operating pilot plants of up to 5 MWe, development of a peaking turbine and of a 50 MWe commercial plant in the UK [5]. LAES plants will benefit from heat recovery from advanced fuel gasifiers [6] to liquefy air, in addition to solar and wind etc. Integrated gasification combined cycle [7] plants with fuel gasifiers have experienced a period of strong growth [8], which was subsequently interrupted by decommissioning and change-over to natural gas due to high carbon release.
Gasifier, prime mover and liquefier performance
The unique cryo-compression [9] air supply economizes both fuel and liquid air consumption in prime movers. In addition cry-compression improves exhaust gas heat recovery by replacement of the organic Rankine cycle with a liquid air cycle. In the interest of promoting grid independence, this post describes only residential size distributed generation plants with cryo-compression prime movers and the neighborhood size infra-structure for supplying sink liquid air and compressed or liquefied fuel; compact vehicles are included at average of two per residence. However, the concept of cryo-compression is applicable to the full size range of mobile and stationary prime movers including commercial and industrial. Power for air liquefication, which previously relied on off-peak grid and intermittent solar and wind, will be supplementary to off-peak electric generation and recovered heat from fuel gasifiers and from prime movers.
The unique cryo-compression [9] air supply economizes both fuel and liquid air consumption in prime movers. In addition cry-compression improves exhaust gas heat recovery by replacement of the organic Rankine cycle with a liquid air cycle. In the interest of promoting grid independence, this post describes only residential size distributed generation plants with cryo-compression prime movers and the neighborhood size infra-structure for supplying sink liquid air and compressed or liquefied fuel; compact vehicles are included at average of two per residence. However, the concept of cryo-compression is applicable to the full size range of mobile and stationary prime movers including commercial and industrial. Power for air liquefication, which previously relied on off-peak grid and intermittent solar and wind, will be supplementary to off-peak electric generation and recovered heat from fuel gasifiers and from prime movers.
Fuel gasifier with carbon capture, air liquefier and electric generation
Growing deployment including conversion and recommissioning of synthetic fuel gasifiers [9] provides opportunity for production of H2 rich syngas and cogeneration of electricity and of liquid air. The carbon capture gasifier delivers high hydrogen syngas in conjunction with in-situ CO2 capture, while recovered heat of burning recirculating char and of discharging product H2 and CO2, raises steam to support gasification.
Cold gas efficiency of wood gasification to H2 is 70% with output of 1.5 lb wood/kWh electric generation. Estimated performance of an exemplary 100 kWt plant case [ ] is based on wood feedstock with limestone sorbent at 630 oC (1165 oF) and air oxidizer, yielding hydrogen content of 150 g/kg (0.33 lb/lb)H2/feedstock while discharging 264 g/kg (0.58 lb/lb) CO2/feedstock for capture.
The gasifier generally operates at steady state, while electric output of engine-generators shifts to provide off-peak power to an air liquefier, with supplemental power from wind and solar, as available.
Production of liquid air is via an advanced liquefier powered by excess gasifier steam or by a cryo-compression turbo-charger operating on combustion of low H2 syngas. The turbo-charger can turn in excess of 200,000 rpm, approximately twice the maximum speed for electric generation, to accommodate minimal air liquefier capacity requirements. Product electric generation is then powered by at least one additional cryo-compression engine operating on turbo-charger exhaust heat via a recovery heat exchanger.
Carbon capture gasifier w/heat recovery to thermo-chem H2
Integration of the fuel gasifier above with a thermo-chemical water splitting reactor provides a "third way" primary heat source to replace problematic solar and nuclear heating for H2 synthesis. This is contingent on development of a viable water splitting process; the most promising being the Sulfur-iodine and Hybrid-sulfur cycles [11]. Operation of a carbon capture gasifier with water splitting is generally as with the non-water splitting configuration above, including syngas generated electricity and production of liquid air. Recovered heat of burning recirculating char and of discharging product H2 and CO2, raises steam to support gasification, while supplemental steam to support water splitting can be generated by heat of tar discharge and solar. Improved performance includes reduction of CO2 for capture.
Integration of the fuel gasifier above with a thermo-chemical water splitting reactor provides a "third way" primary heat source to replace problematic solar and nuclear heating for H2 synthesis. This is contingent on development of a viable water splitting process; the most promising being the Sulfur-iodine and Hybrid-sulfur cycles [11]. Operation of a carbon capture gasifier with water splitting is generally as with the non-water splitting configuration above, including syngas generated electricity and production of liquid air. Recovered heat of burning recirculating char and of discharging product H2 and CO2, raises steam to support gasification, while supplemental steam to support water splitting can be generated by heat of tar discharge and solar. Improved performance includes reduction of CO2 for capture.
Thermo-chemical water splitting operates on a selected cycle below the gasifier outlet temperature of ~1100 oC (2010 oF). Over 300 cycles have been evaluated for H2 yield, however less than 10 candidate cycles have been qualified for development. The example cycle of this post is the Hybrid Sulfur Cycle [13], which is under advanced development by Savannah River National Laboratory. It requires heat input for sulfur decomposition at ~90 bar and 850 oC (1560 oF). Recycling char from the steam gasification reaction above slag liquefaction temperature of 1200 oC (2200 oF) delivers syngas in parallel with thermo-chemical H2. Approximately 70% of the gasifier/thermo-chemical reactor gas yield is thermo-chemical H2, based on 120 kJ/molH2 [14] superheat in the sulfuric acid decomposition reaction. Process reactions downstream of the thermo-chemical reactor are separation of SO2 to yield O2, electrolysis of SO2 to yield H2, and concentration of H2SO4 for return to the high temperature thermo-chemical reactor.
Stationary prime mover w/cryo-compression
The stationary cryo-compression engine operates with phase change storage, in which sub-cooling of working fluid entering the cryo-compressor is by liquefied air or nitrogen, via a cryo-recuperator. Highest performance is with a hybrid arrangement of a solid oxide fuel cell and wood gasifier or other high temperature source connected to a heat recovery cycle with an air turbine (AT) or a positive displacement piston expander (PE). Source temperature of the gasifier is up to ~1100 oC (2012 oF) to maintain margin over tar cracking temperature. Source temperature of the solid oxide fuel-cell is up to ~1000 oC (1832 oF) imposed by the solid electrolyte. These temperatures are compatible with both the AT and PE inlet gas temperature limit of ~870 oC (1600 oF). Sink temperature is -190 oC (-315 oF).
Small stationary CCPE/TC
Small stationary CCIC/TC
Small stationary CCGT/TC
Small stationary CCFC/TC
An exemplary cryo-compression solid oxide H2 fuel-cell and a reciprocating piston expander heat recovery unit is considered for low stationary generating capacity, down to ~4kWe. Cryo-compression and recovered fuel cell exhaust energy boosts thermal efficiency from ~50% for the FC to ~75% for the FC/TC. The cryo-PE operates on a modified Ericsson cycle adapted to quasi-isothermal compression by liquid air injection. Alternate positive displacement expanders include scroll, screw, vane and gerotor [14].
The exemplary FC/TC develops 4.0 kWe (FC = 2.7 kWe, TC = 1.3 kWe) at 8000 rpm. Compression work is charged to the TC because the FC requires minimal compression in the hybrid arrangement. H2 consumption at 100% power is 0.034 kg/kWh (0.076 lb/kWh) with a lqa/H2 mass ratio of 36, based on isentropic efficiency = 70% and pressure ratio = 10. The cryo-sink reduces TC fuel consumption by ~60% as compared to a normally aspirated Ericsson cycle engine, while reducing compressor/turbine work and pressure ratio. PE compression work is ~25%, as with ambient air intake. Emissions are reduced in proportion to fuel consumption and dry ice may be deposed from engine exhaust for sequestration or liquefier pre-cooling.
Distributed generation FC/AT
An exemplary cryo-compression solid oxide H2 fuel-cell and an AT heat recovery unit is considered for larger stationary generating capacity, down to ~250 kWe. An advantage of the AT with cryo-sink/fuel-source storage is convertibility of available micro-turbines and turbo-chargers to cryo-compression. Cryo-compression and recovered fuel cell exhaust energy boosts FC thermal efficiency from ~50% to ~75% for the FC/AT. The cryo-AT operates on a modified Brayton cycle adapted to quasi-isothermal compression by liquid air injection. AT generating capacity is above ~ 50 kWe due to rotor speed limitation.
The exemplary FC/AT develops 250 kWe (FC = 180 kWe, AT = 70 kWe) at 100,000 rpm. Compression work is charged to the AT because the FC requires minimal compression in the hybrid arrangement. H2 consumption at 100% power is 0.036 kg/kWh (0.080 lb/kWh) with a lqa/H2 mass ratio of 24, based on isentropic efficiency = 85%, pressure ratio = 4 and recuperator effectiveness = 90%. These conditions are representative of a commercially available micro-turbine, with speed limited to 100,000 rpm. Turbine compression work is ~25%, as with ambient air intake. Emissions are reduced in proportion to fuel consumption and dry ice may be deposed from engine exhaust for sequestration or liquefier pre-cooling.
Stationary air liquefier
Phase change using liquid air at - 193 oC (-315 oF) is unique among storage concepts, providing enhanced prime mover performance by lowering heat sink temperature. An air liquefier supplies liquid air for cooling the cryo-compressor of a prime mover. The ref. liquefier is a low pressure Kapitza compression/expansion machine and higher performance alternates include magneto-caloric and thermo-acoustic concepts.
Power to drive air liquefiers associated with fuel-cell and fuel gasifier is primarily from off-peak generation and recovered energy of the fuel-cell and gasifier. These power sources may be supplemented by solar, wind and other available sources. Power to independent air liquefiers is also from solar, wind and other sources as available. Solar drive of the liquefier compressor is via a Rankine steam cycle or photo-voltaic panels. Potential for enhanced wind drive is by structure induced aspiration [15], increasing wind capture by over 400% as compared to a conventional wind turbine. The liquefier compressor is coupled to a fan, which is under ~5 differential velocity heads between impact pressure and suction in the wake of leading edges of a building, wall or fence.
Motor vehicle prime mover w/cryo-compression
The cryo-compression engine for fuel cell heat recovery in motor vehicles operates with phase change storage, in which sub-cooling of working fluid entering the compressor is by liquefied air or nitrogen, via a cryo-recuperator. Both gas turbine and positive compression engines, such as a reciprocating piston expander, are adaptable to cryo-compression, however high air flow and rotor speed limitation preclude the gas turbine from vehicle application below ~300 HP. The cryo-PE operates on a modified Ericsson cycle adapted to quasi-isothermal compression by liquid air injection. Source temperature is up to constant 1090 oC (2000 oF) in the gas turbine and instantaneous 1650 oC (3000 oF) in a piston expander, with sink temperature of - 190 oC (-315 oF). Alternate positive displacement expanders include scroll, screw, vane and gerotor [15].
Compact vehicle FC/PE
An exemplary H2 fueled SOFC with cryo-compression and a reciprocating PE heat recovery unit is considered for smaller vehicle application ranging from a compact car with AfCd = 0.6 m2 (6.5 ft2) and wgt. = 1360 kg (3000 lb) to a mid-size pick-up truck with AfCd = 1.3 m2 (14 ft2) and wgt. = 2700 kg (6000 lb). Fuel and liquid air consumption and capacity are estimated at continuous highway speed of 113 km/h (70 mph) with power ranging from 22 HP (14 aero, 8 rolling) at 4300 rpm for the compact to 48 HP (32 aero, 16 rolling) at 2500 rpm for the truck. Under these conditions H2 consumption of both the car and truck is 0.024 kg/HP (0.053 lb/HP) with a lqa/H2 mass ratio of 38. Results are based on PE isentropic efficiency = 70% and pressure ratio = 10. Cryo-compression and recovered fuel cell exhaust energy boosts thermal efficiency from ~50% for the FC to ~75% for the FC/PE.
Full size vehicle FC/AT
An exemplary H2 fueled SOFC with cryo-compression and an AT heat recovery unit is considered for larger vehicle application in a full size car or truck with AfCd = 1.3 m2 (14 ft2) and wgt. = 2700 kg (6000 lb). An advantage of the AT with cryo-sink/fuel-source storage is convertibility of available micro-turbines and turbo-chargers to cryo-compression. Cryo-compression and recovered fuel cell exhaust energy boosts FC thermal efficiency from ~50% to ~75% for the FC/AT. The cryo-AT operates on a modified Brayton cycle adapted to quasi-isothermal compression by liquid air injection. AT power is above ~ 100 HP due to rotor speed limitation.
The exemplary FC/AT develops 50 HP (FC = 36 HP, AT = 14 HP) at 113 km/h (70 mph). Compression work is charged to the AT because the FC requires minimal compression in the hybrid arrangement. H2 consumption is 0.03kg/HP (0.07 lb/HP) with a lqa/H2 mass ratio of 20, based on isentropic efficiency = 85%, pressure ratio = 3 and recuperator effectiveness = 90%. These conditions are representative of a commercially available micro-turbine, with speed limited to 100,000 rpm. Turbine compression work is ~25%, as with ambient air intake. Emissions are reduced in proportion to fuel consumption.
Motor vehicle air liquefier
An on-board air liquefier supplies liquid air for cooling the cryo-compressor of the prime mover. Supplementary liquid air from an external source may be added, as required. The ref. liquefier is a low pressure Kapitza compression/expansion machine and higher performance alternates include magneto-caloric and thermo-acoustic concepts. .
Recovered Vehicle Energy
Regenerative braking is a well developed technology, which can operate to drive the compressor of an on-board vehicle air liquefier. An estimated 25% and 50% of combined rolling and aerodynamic energy is recoverable to cryo-PE and cryo-GT powered vehicles, respectively. Higher recovery of the cryo-GT is attributed to low cryo- GT pressure ratio.
Nomenclature
Af vehicle frontal area
AT air turbine
Cd drag coefficient
FC fuel cell
GT gas turbine
IGCC integrated gasification combined cycle
LAES liquid air energy storage
Lqa liquid air
PE piston expander
SOFC solid oxide fuel cell
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